A. CORROSION:
| Corrosion
is an electrochemical reaction converting the
metal into its oxide. Corrosion requires an
anode, cathode & an electrolyte. The metal
acts as an anode & cathode while water acts
as an electrolyte. |
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FOLLOWING
FACTORS AFFECT THE RATE OF CORROSION:
a) Metallurgy of the system
b) PH of circulating water
c) Dissolved gases
d) Dissolved and suspended solids
e) Water velocity
f) Temperature
g) Microbial growth |
Under Deposit Corrosion View after Cleaning
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IMPORTANT
TYPES OF CORROSION WHICH ARE FREQUENTLY FOUND IN
COOLING WATER SYSTEMS:
a) General type of corrosion
b) Localized corrosion
c) Corrosion due of velocity
d) Corrosion due to mechanical stress
| HOW
DO THE CORROSION INHIBITORS WORKS IN COOLING
WATER TREATMENT |
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The Corrosion Inhibitors present in Water Treatment Chemicals form the passivation film on the metal surface. Thus this surface is impervious to ions transfer or oxygen attacks and this barrier is arresting the corrosion.
As the film is very thin, it in no way affects the heat transfer. |
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B. SCALING:
Scaling is defined as the hard and adherent deposits formed due to precipitation of sparingly soluble salts in water. The most commonly occurring scalants in cooling water systems are carbonates, sulphates, phosphates and silicates of calcium and magnesium. |
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Scale Deposition:-
The scale deposits give rise to the following problems in cooling water systems:
a) Reduced heat transfer decreasing the heat transfer efficiency.
b) Increased pressure drop on water side.
c) Under Deposit Corrosion. |
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Following
are the factors which affect the scaling in
cooling water system:-
a) Temperature
b) pH
c) Solubility
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HOW DOES THE ANTISCALANT WORK?
For scaling to form, the crystal of scaling salt has to grow sufficiently in size.
When the crystal is growing the ANTISCALANT is absorbed on the crystal, blocking the growth site. Thus the crystal cannot grow in size. Even it the crystal grows, it is imperfect and the structure is very fluffy. The small or fluffy crystals cannot form hard deposits hence Scaling does not occur. In presence of ANTISCALANT, the metal surface remains free from Scaling.
The ANTISCALANTS inhibit scale formation by increasing the solubility of scalants in water and help to remain higher levels of scalants in dissolved form.
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Electron Photomicrographs showing scale without antiscalant
Electron Photomicrographs showing scale with antiscalant
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| FOULING : |
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| Fouling is the deposition of suspended particles. The particulate matter generally accumulates at low velocity areas in the cooling water system. If cooling water is on the shell side of the heat exchanger then because of low velocity the fouling material settles on the shell side |
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Fouling Deposition:-
THE POTENTIAL FOULANTS IN COOLING WATER SYSTEMS ARE AS FOLLOWS:
a) Dust and silt.
b) Corrosion Products.
c) Sand
d) Natural organics
e) Microbial matter
THE FOLLOWING FACTORS AFFECT THE FOULING OF THE SYSTEM:
a) Water characteristics
b) Temperature
c) Water velocity
d) Microbial growth
| HOW DOES THE
ANTIFOULANT WORK? |
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For fouling to take place, small suspended particles have to come together to form Agglomerate. Most of the suspended matter is in the colloidal state and have a small electric charge on them.
ANTIFOULANT is polymeric in nature and when it is absorbed on suspended particles, it will increase the negative charge on the particle. As like charges repel, the suspended particles are thus kept apart, preventing their agglomeration.
The particles thus stay dispersed in the water and are prevented from depositing and fouling the system.
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